Analysis of the results of radioecological monitoring of the Ob and Irtysh rivers in the area of their confluence

Relevance. Long-term radiation impact of nuclear fuel cycle enterprises in the Urals and Siberia on the Ob and Irtysh rivers. The results of continuous 10-year radioecological monitoring of aquatic ecosystems made it possible to obtain reliable data on the dynamics of the radiation situation on the Ob and Irtysh rivers at their confluence. An assessment is given of the contribution of the Ob and Irtysh waters to the integral flow of artificial radionuclides into the Lower Ob and further into the Kara Sea, as well as the role of the components of the aquatic ecosystems of the studied rivers in the migration of the most significant isotopes. Aim. To assess the radiation situation on the Ob and Irtysh rivers in the area of their confluence based on an analysis of data from long-term radioecological monitoring. Objects. Surface water, bottom sediments and floodplain soils of the Ob and Irtysh rivers in the area of their confluence. Methods. Samples of freshwater ecosystem components were collected in 2004–2013 at sampling locations of the established radioecological monitoring network. 90Sr, 137Cs and 239,240Pu were determined in surface water, bottom sediments and floodplain soils. Sampling and sample preparation for assessing the content of artificial radionuclides were carried out according to certified methods. The specific activity of radionuclides in samples was measured using gamma spectrometric, radiochemical, and radiometric methods. Results. Over the 10-year observation period, the maximum values of specific activity of 137Cs, 90Sr and 239,240Pu in the Ob and Irtysh rivers were two to three orders of magnitude lower than the intervention level established in NRB-99/2009. Thus, the specific activity of 137Cs in the Ob river water varied from 0.1 to 6.4 mBq/L, in the Irtysh from 0.1 to 10.6 mBq/L. The content of 90Sr in water was somewhat higher and was within the range of 5 to 98 mBq/L and from 10 to 92 mBq/L, respectively. The range of changes in 239,240Pu was 0.02–0.2 mBq/L in the Ob and 0.01–0.12 mBq/L in the Irtysh. Two time periods were identified, associated with an increase (2004–2007) and then a decrease (2008–2013) in the specific activity of artificial radioisotopes in the water of both rivers. The integral average annual runoff of radionuclides into the Lower Ob and further into the Kara Sea over the observation period was (TBq): 137Cs – 0.49, 90Sr – 4.3 and 239,240Pu – 16.3.